Merlin, Merlin Legend, Partner and Partner ACS were in every dentist’s office, insurance agency and pizza place in America for twenty years.
What callers heard
The Merlin Legend manual tells installers to pick the jack “reserved for connection of a music source, such as a radio, tape player, or stereo system.” Only one music jack is allowed per system. There was no built-in tune. With nothing plugged in, hold was silent.
The same manual warns about copyright. Retransmitting licensed music to callers needs permission, and a radio station’s broadcast license doesn’t cover a business piping it down phone lines. Plenty of offices plugged in a radio anyway, which is why “hold music” sometimes meant a local DJ and a mattress ad.
The clip above is a stand-in: a public-domain tune on a worn loop cassette, the other common source back then.